Impact of particle-attached microbial denitrification on N2O production in an agricultural-urban watershed

Abstract

Anthropogenically influenced rivers are key hotspots for nitrous oxide (N₂O) emissions. However, the seasonal and spatial heterogeneity of N₂O emissions in subtropical riverine systems, particularly the role of particle-attached microbes (PAM) in regulating N₂O production, remains poorly understood, contributing to uncertainties in global N₂O estimates. This study investigates the potential impacts of PAM-driven nitrogen transformations on N₂O production in the Dongjiang River under agricultural and urban influences. Water samples collected during the wet and dry seasons were analyzed for N₂O concentrations, dual nitrogen-oxygen isotopes (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻), and metagenomic sequencing of PAM. All samples exhibited N₂O supersaturation, with emissions significantly higher in the dry season than in the wet season. A linearly positive δ¹⁵N–δ¹⁸O correlation, accompanied by lower NO₃⁻ in the bottom layers than the surface layers in the dry season indicates active denitrification, leading to elevated N₂O concentrations. PAM-driven denitrification was identified as the dominant nitrogen transformation process, supported by higher abundances of denitrification genes (nirKS, norBC, nosZ) relative to nitrification genes (amoABC). Despite aerobic water column conditions, low-oxygen microhabitats around suspended particles facilitated N₂O production. A significantly positive correlation (p < 0.05, R² = 0.42) between N₂O concentrations and the nirK/nosZ gene ratio suggests that gene expression imbalances contributed to net N₂O accumulation. Additionally, the downstream urban area exhibited lower DO and higher DOC levels, enhancing denitrification and increasing N₂O production by 4.7 % compared to the upstream agricultural region. Seasonal differences further influenced N₂O dynamics: higher DOC/NO₃⁻ ratios in the dry season promoted heterotrophic denitrification, while elevated temperatures in the wet season favored complete denitrification, reducing N₂O emissions. These findings provide critical insights into PAM-driven nitrogen cycling, informing strategies for mitigating N₂O emissions and managing nitrogen pollution in subtropical riverine systems.